Reason for that is Googles and Facebooks ongoing hunt for personal information, ignorance of Privacy and their business model to serve you targeted ads. Its not them alone, though! I for myself decided not to help them in doing so and I just found a wonderful article which summarizes the problem way better then I would be able to.
So, I urge you to head over to https://motherboard.vice.com/en_us/article/xwjden/targeted-advertising-is-ruining-the-internet-and-breaking-the-world.
Read that article. Understand the message. Understand the consequences. And act. Even the smallest change in your behaviour, the decision to buy a pricey (or an used) iPhone next time or to install an Ad-blocking tool could change the world a little bit.
Read. Understand. Act.